This invention relates to a reactive ion etching (RIE) apparatus and, more particularly, to an improved pallet in electrical communication with the cathode therein for providing high etch uniformity across the entire pallet surface.
In the manufacture of semiconductor devices considerable interest exists in employing dry processing techniques for patterning workpieces such as semiconductor wafers. The interest in dry etching techniques stems from their generally better resolution and improved dimensional and shape control capabilities relative to etching using wet chemicals. Among the dry etching techniques, reactive ion etching has been favored, especially with ultrafine resolution demanded by devices of the very large scale integration (VLSI) type because of its compatibility to achieve high etch rates and extremely well-defined edges on etched materials.
The requirements for high volume handling of semiconductor wafers has resulted in the use of multiple wafer batch RIE systems. In order to enhance the throughput and to simultaneously process a large number of wafers without human intervention the RIE systems have been automated. In an automated RIE system the wafers are automatically transported, loaded, processed and unloaded.
The RIE system typically consists of a container wherein are positioned an anode and a cathode. The cathode is negatively biased relative to the anode by means of a radio frequency (RF) potential. The surface to be etched is covered by the suitable mask and then placed directly on the cathode or on a pallet which is electrically and mechanically connected to the cathode. A chemically reactive gas such as CF.sub.4 is introduced into the container maintained at a pressure, typically, in the millitorr range. The electric field established in the region between the anode and the cathode will dissociate the reactive gas forming a plasma therein. Chemically reactive gas ions in the plasma are attracted to the cathode and thereby impinge on the surface to be etched. Apparently, the surface is etched both by chemical interaction with the active ions and by the momentum transfer of the ions striking the surface. By virtue of the electric field attracting the ions to the cathode, the ions strike the surface to be etched predominantly in a direction perpendicular to the surface so that the process produces well-defined vertically etched side walls.
In order to maximize the etch rates and effectiveness of etching, the gaseous plasma is generally confined to the small reaction volume between the cathode and the anode. One way of accomplishing this relatively small reaction volume is by utilizing a flat disc-shaped cathode and a cylindrical cap-shaped anode covering the cathode and electrically insulated from the cathode. In this arrangement, since the cathode diameter is smaller than the anode diameter the inside walls of the cylindrical anode is spaced from the cathode plate. Consequently, although the electric field in the central portion of this reaction volume is uniform and perpendicular to the cathode surface, the field along the outer edge areas of the cathode varies with radial position and is not perpendicular to the cathode surface. As a result of the nonuniformity in the electric field in the reaction volume to etch rate of wafers placed at various radial positions on the cathode is not uniform, the etch rate at the center of the cathode being greater than that at the outer edge. Also, since the electric field along the outer edge of the cathode is not perpendicular to the cathode surface, the etch profile of the wafers placed along the outer edge is poor.
A similar lack of uniformity in the etch rate and absence of consistently well-defined edges on etched materials occurs in automated RIE systems which utilize a pallet typically made of silicon for transporting the wafers into or out of the reaction volume. The pallet diameter is required to be somewhat larger than the diameter of the cathode in order that, for example, at the termination of the etching process, the pallet may be gently lifted up from the cathode and carried out of the process chamber by the transfer carrier. In this removable pallet RIE arrangement, since the pallet's diameter is larger than that of the cathode, the electric field is uniform and perpendicular to the pallet surface only over the portion of the pallet that is in direct contact with (or overlaps) the cathode and within approximately 1 inch from the cathode's perimeter. In other words, the electric field along the ring portion of the pallet from the pallet's outer edge to approximately 1 inch interior to the cathode's outer edge is nonuniform and the field lines are inclined at various angles. As a result of this variation of electric field strength and direction, the etch rate of the wafers mounted along the outer edge of the pallet is nonuniform and the profiles of their etched surfaces unacceptably poor.
It would appear that one solution to the above problem is to mount a metallic plate on the cathode thereby extending the cathode as illustrated in FIG. 1 of U.S. Pat. No. 4,283,249 which is assigned to IBM Corporation, the present assignee. Such an arrangement, although extends the area of the cathode over which the electric field is uniform and perpendicular to the cathode surface, suffers from sputtering of the cathode material (even at very low power densities) which will diffuse back on the surfaces being etched and introduce defects therein. Another disadvantage is the so-called loading problem created in the reaction volume due to the inability of the metal cathode to maintain a uniform and high density of reactive ion species.
Another solution is repositioning the wafers to be etched toward the center of the pallet/cathode or, alternatively, improving the electric field in the pallet/cathode edge areas so that the field is perpendicular to the cathode/pallet surface. However, both these approaches require extensive modification, particularly in the case of fixed automated handling RIE systems, which is prohibitively expensive, to maintain the high throughput demanded of such systems.
Accordingly, it is an object of this invention to provide a RIE apparatus which allows an enhanced etch uniformity across the workpiece.
It is a further object of this invention to provide a RIE apparatus which enables excellent etch uniformity across the entire cathode surface.
It is still another object of this invention to provide a RIE apparatus which enables excellent etch uniformity over the entire batch of workpieces which are in electrical communication with the cathode therein without introducing contaminants into the etched workpieces.